Block copolymers of polysiloxanes and polyalkylene ethers(polyethers) are known in the art and have been prepared by various methods. They can be used as surfactants or as surface active monomers to modify the surface energy of polymers. Although a few of them are truly linear block copolymers, many of them would be more properly described as graft or “comb” copolymers.
Linear block copolymers of polysiloxanes and polyalkylene ethers have been prepared by reaction of difunctionally terminated polysiloxane oligomers with dihydroxyl terminated polyalkylene ethers. Examples of suitably terminated polysiloxanes used to prepare these block copolymers include acetoxy, alkoxy, and dialkylamino-terminated polysiloxanes. (For an overview on the synthesis of these polyether/polysiloxane block copolymers, see A. Noshay and J. E. McGrath; “Block Copolymers; Overview and Critical Survey”; Academic Press, New York, 1977; pp 400-401.)
The aforementioned polysiloxane-polyalkylene ether linear block copolymers are all synthesized by condensation polymerization, in which the terminal group on the polysiloxane oligomer is displaced by the hydroxyl group of the polyalkylene ether to produce the desired block copolymers having an Si—O—C linkage between the polysiloxane and polyalkylene oxide blocks of the copolymer. A low molecular weight by-product, resulting from displacement of the end-group that was previously attached to the polysiloxane block, is liberated during the polycondensation reaction. This by-product must either be removed from the polymer in an additional processing step, or allowed to remain in the block copolymer. In many polymer applications, these low molecular weight components can act as plasticizers, which detrimentally affect the polymer properties, and/or can slowly migrate out of the polymer over time presenting potential safety issues or detrimental performance (fogging or oily materials rising to the surface). Furthermore, as will be appreciated by those skilled in the art, polycondensations of this type are known to produce broad molecular weight distributions. The breadth of the molecular weight distribution is typically characterized by the polydispersity index, which is the ratio of weight average molecular weight (Mw) to number average molecular weight (Mn). For condensation polymerizations, Flory's theory has been used to derive a theoretical ratio of Mw/Mn (polydispersity) of 1+p, where p is the extent of reaction. For high polymers, the extent of reaction approaches 1, so that the expected polydispersity is ˜2.0, which has been experimentally verified for a number of condensation polymerizations. See, e.g., G. Odian, “Principles of Polymerization” 3rd edition, pp. 85-87, John Wiley and Sons, NY, 1991.
In order to overcome the aforementioned difficulties associated with the synthesis of polyether/polysiloxane block copolymers, Takeyasu et. al. (EP 0 485 637 B1; Asahi Glass Company Ltd.) describes preparation of PET/PDMS copolymers using Double Metal Cyanide (DMC) catalysts to alkoxylate hydroxyalkyl-terminated polydimethylsiloxanes. However, in their work, the use of silanol-terminated starting polydialkylsiloxanes is not disclosed.
U.S. Pat. No. 3,182,076 describes the preparation of organopolysiloxanes with carboxyalkyl terminal groups (not polymer chains) bound to the ends of the siloxane chains. Thus they aren't really block copolymers, but rather organopolysiloxanes terminated with carboxyalkyl groups. The linkage binding the organosiloxane components with the carboxyalkyl terminal groups are of the Si—C type. True block copolymers are not disclosed, nor is an Si—O—C linkage.
There exists therefore a need for polysiloxane-polyalkylene ether linear block copolymers having narrow polydispersity (<1.5) that do not contain low molecular weight by-products.